1. Field of the Invention
The present invention relates to a color sensor using a diffraction grating for color recognition or colorimetry on a subject to be tested, in particular, a color sensor which is suitable for performing colorimetry on toner or printing media in an image forming apparatus.
The present invention may be applied not only to the image forming apparatus but also to a spectral colorimetric apparatus such as a white balance sensor for a digital camera, and other colorimetric apparatus.
2. Description of the Related Art
In an image forming apparatus for forming a color image through an electrophotographic process, a deviation in color tone may occur because of color mixture of toner. The same problem occurs, not only in the electrophotographic type image forming apparatus but also in a general image forming apparatus for forming a color image such as an ink-jet type image forming apparatus.
For example, Japanese Patent Application Laid-Open No. H09-160343 proposes a method of measuring a spectral reflection light amount of a toner image using two different spectral filters and correcting an image signal based on the result of the measurement.
Further, in order to determine the color tone at higher precision, it is necessary to increase the number of spectral wavelength bands to at least three, which corresponds to the number of primary colors. When the number of wavelength bands may be further increased, the color tone may be determined at higher precision.
In order to increase the number of wavelength bands, there have been proposed a large number of diffraction spectrometers for performing spectral measurement, for example, in Japanese Patent Application Laid-Open No. 2000-298066.
When the diffraction spectrometer is used, there have been problems that still remain to be solved.
Up to now, a Rowland type diffraction spectrometer has been generally used. In the Rowland type diffraction spectrometer, dispersed light from a subject to be tested, which is illuminated by an illumination optical system, is incident on an incident slit. A light beam entering the incident slit is spectrally separated by a concave reflection type diffraction optical element and then obtained as a spectral intensity distribution by a one-dimensional array detector.
In a visible light region, an Si photo diode array is generally used as the one-dimensional array detector. A detector using the Si photo diode array has a maximum sensitivity in a near-infrared region but the sensitivity reduces as a wavelength shortens. Therefore, for example, there is a problem that a several-time detection sensitivity difference between a short-wavelength side and a long-wavelength side occurs in the visible light region.
In order to downsize the diffraction spectrometer, it is also necessary to reduce a size of an illumination light source. A halogen lamp or a xenon lamp, which has conventionally been used, has a problem in terms of necessary space and a problem of heat generation. In recent years, a high-intensity white LED of a type in which a fluorescence body is excited by an ultraviolet LED to obtain white light is used as an alternative to a light source including the halogen lamp or the xenon lamp. Even in an ultraviolet-excitation type white LED of which a color rendering property is said to be high, a spectral intensity in the short-wavelength region is significantly lower than a spectral intensity at the center wavelength, and hence the insufficient sensitivity of the one-dimensional array detector cannot be compensated. In order to effectively illuminate the subject to be tested, with a uniform amount of light based on light from the white LED which is a light source, a light guide is required. In order to reduce a size of the light guide, a transmission type element having a total-reflective surface and a refractive surface is desired. The element has a complicated shape and thus is desirably manufactured by plastic injection molding. However, a plastic material (polymethylmethacrylate (PMMA) or polycarbonate) generally has a low transmittance in the short-wavelength region, and hence the insufficient sensitivity of the one-dimensional array detector cannot be compensated.
Meanwhile, when spectral separation is performed using a diffraction grating, for example, first-order diffracted light is used. However, high-order undesirable diffracted light is actually generated. When a use wavelength band in the spectrometer is, for example, in a range of 350 nm to 750 nm, second-order diffracted light for a wavelength band of 350 nm to 375 nm overlaps with first-order diffracted light in a wavelength band of 700 nm to 750 nm on the detector, to thereby cause color mixture. Therefore, there is a problem that a difference occurs between the original chromaticity of the subject to be tested and detected chromaticity.
Up to now, for example, a method of cutting undesirable diffracted light using a high-order diffracted light cut filter has been employed for the problems. However, the high-order diffracted light cut filter is expensive, it is essential to reduce a size of the cut filter to be incorporated into an image forming apparatus, and there is another disadvantage in reducing the number of parts for needs to reduce an apparatus size.